1. Field of the Invention
This invention relates to a method and an apparatus for measuring the dynamic response characteristics, including the gain characteristics and the phase characteristics, of a shock accelerometer of the type used for measurements in drop impact tests conducted on containers for transporting radioactive wastes, impact tests conducted on structures, vehicle collision tests and the like.
2. Prior Art Statement
Shock accelerometers are employed in such wide-ranging fields of industrial measurement as collision tests conducted on structures, drop impact tests, stress analysis tests and vibration analysis tests.
As a method for calibrating an shock accelerometer, there is known that set out in ISO 5347, which is a method using a combination of a vibrator and a laser interferometer. With this conventional method, however, it is difficult to carry out high-precision calibration because as the frequency of the vibrating table becomes higher, the vibration amplitude becomes smaller and the waveform deviates from sinusoidal. While high frequencies excitation is possible by using a resonant vibrating table, the excitation frequency that can be produced is determined by the particular vibrating table so that where it is desired to measure the characteristics of an accelerometer over a wide frequency range, it becomes necessary to use a large number of resonant vibrating tables with different excitation frequencies. This leads to unreliable calibration and is also uneconomical. Moreover, although the conventional calibration method is intended for use at vibration accelerations of 100 m/s.sup.2 or less, it is in fact used for calibration at higher vibration accelerations because there is no other method available. The reliability of the detected values is thus highly doubtful.
Since an accelerometer can be modeled as an oscillating system consisting of a spring and a mass, it has its own resonant frequency. Because of this, the gain (sensitivity) of an accelerometer is a function of the frequency of the input acceleration. Generally, the gain is constant in the low frequency region but increases as the frequency of the input acceleration approaches the resonant frequency. As the accelerometer can be used only in the region of constant gain, the frequency at which the gain increases by a prescribed rate is defined as the upper limit frequency at which the accelerometer can be used. The relationship between the frequency of the input acceleration and the gain will be referred to here as the gain characteristics.
In any dynamic system, of which the accelerometer is only one example, the system delay increases as the timewise variation in the input signal becomes faster, causing a phase lag between the input signal and the output signal. Therefore, similarly to the case of the gain, the frequency at which the phase lag comes to exceed the tolerable limit is defined as the upper limit frequency for the phase.
Elastic waves and plastic waves are generated in the interior of an object to which an accelerometer is attached when the object is subjected to an impact. In the measurement of shock acceleration for evaluation of breaking, the acceleration output signal is sometimes passed through an electrical filter for removing the effect of the elastic waves. Where the phase of the filter output signal has to be taken into consideration, it is indispensable to have prior knowledge of the relationship between the phase lag and the frequency of the accelerometer output signal itself, that is to say, of the phase characteristics. Also, in cases where the signal output by the accelerometer is to be used as a trigger signal and the timing thereof is subject to severe conditions, it becomes impossible to use phase compensation and other such control techniques unless the phase characteristics are known.
Thus for calibrating an shock accelerometer it is necessary to determine both the gain characteristics and the phase characteristics.